Identification of a Novel Vasodilatory Octapeptide from the Skin Secretion
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molecules Article Identification of a Novel Vasodilatory Octapeptide from the Skin Secretion of the African Hyperoliid Frog, Kassina senegalensis Qiang Du 1,†, Hui Wang 1,*,†, Chengbang Ma 2, Yue Wu 2, Xinping Xi 2,*, Mei Zhou 2 ID , Tianbao Chen 2 ID , Chris Shaw 2 and Lei Wang 2 1 School of Pharmacy, China Medical University, Shenyang 110001, Liaoning, China; [email protected] 2 Natural Drug Discovery Group, School of Pharmacy, Queen’s University, Belfast BT9 7BL, Northern Ireland, UK; [email protected] (C.M.); [email protected] (Y.W.); [email protected] (M.Z.); [email protected] (T.C.); [email protected] (C.S.); [email protected] (L.W.) * Correspondence: [email protected] (H.W.); [email protected] (X.X.); Tel.: +86-24-2325-6666 (H.W.); +44-28-9097-2200 (X.X.); Fax: +86-2325-5471 (H.W.); +44-28-9094-7794 (X.X.) † These authors contributed equally to this work. Received: 5 July 2017; Accepted: 19 July 2017; Published: 19 July 2017 Abstract: The defensive skin secretions of amphibians continue to be an excellent source of novel biologically-active peptides. Here we report the identification and pharmacological activity of a novel C-terminally amided myotropic octapeptide from the skin secretion of the African hyperoliid frog, Kassina senegalensis. The 8-amino acid peptide has the following primary structure: WMSLGWSL-amide and has a molecular mass of 978 Da. The primary structure and organisation of the biosynthetic precursor of WL-8 amide was successfully deduced from cloned skin secretion-derived cDNA. The open-reading frame encoded a single copy of WL-8, located at the C-terminus. Synthetic WL-8 amide was found to cause relaxation of rat tail artery smooth muscle with an EC50 of 25.98 nM. This peptide is unique in terms of its primary structure and is unlike any other peptide previously isolated from an amphibian source which has been archived in the NCBI database. WL-8 amide thus represents the prototype of a novel family of myotropic peptide from amphibian defensive skin secretions. Keywords: amphibian; skin secretion; peptide; smooth muscle 1. Introduction The skin secretions of anurans (frogs and toads) have long been established as an excellent source of structurally-unique peptides which can be used as lead compounds in research into novel therapeutic agents [1,2]. These peptides have been honed through natural selection and physiological adaptations to aid the survival of the organism over millions of years [3]. A wide range of pharmacologically-active peptides are present in skin secretions. These peptides may exert some biological effect or even be highly toxic to would-be predators [3]. An example which demonstrates the physiological effects these peptides can have on predators has been reported in a study of the garter snake. The defensive secretions produced by Xenopus can cause oral dyskinesias in the snake, such that it cannot physically close its mouth [4]. This effect, if reproduced on similar predators in Nature, would certainly permit the frog to escape. Many of the peptides isolated from amphibians exert their pharmacological functions by acting on specific targets within the body, for instance at G protein-linked receptors. This targeting ability of peptides makes them very interesting molecules in the search for potential drug candidates. The pharmacologically-active peptides found in the skin secretions of amphibians are often found to Molecules 2017, 22, 1215; doi:10.3390/molecules22071215 www.mdpi.com/journal/molecules Molecules 2017, 22, 1215 2 of 9 Molecules 2017, 22, 1215 2 of 8 befunctions analogues in humans of the naturally-occurring and in other vertebrates. regulatory Some are or neuropeptideseven identical to which their endogenous carry out physiological mammalian functionscounterparts in humans such as and thyrotropin-releasing in other vertebrates. hormon Some aree (TRH) even identical[5] and the to theircanonical endogenous bradykinin mammalian peptide, counterpartsbut most are suchnot identical as thyrotropin-releasing though they do hormonehave highly-conserved, (TRH) [5] and thebiologically-active canonical bradykinin regions peptide, within buttheir most structures are not [6]. identical This latter though group they includes do have highly-conserved,bombesin, which is biologically-active an analogue of gastrin-releasing regions within theirpeptide structures in humans [6]. and This also latter the group caeruleins, includes which bombesin, are similar which to gastrin is an analogue and cholecystokinin of gastrin-releasing found in peptidehumans in [7]. humans and also the caeruleins, which are similar to gastrin and cholecystokinin found in humansFrog [7 ].skin secretion is a complex mixture of proteins and peptides which have a variety of pharmacologicalFrog skin secretion effects such is a as complex neurotoxicity mixture and of myotoxicity. proteins and Many peptides proteins which and havepolypeptides a variety have of pharmacologicalbeen isolated from effects amphibian such asskin neurotoxicity which exhibit and a myotoxicity.contractile effect Many on proteinsrat bladder and smooth polypeptides muscle. haveHowever, been the isolated effects fromof low amphibian molecular-weight skin which peptides exhibit on such a contractile smooth muscles effect onare ratnot bladderfully understood. smooth muscle.Here, However, the discovery the effects of a prototype of low molecular-weight low molecular mass peptides peptide on suchfrom smooththe skin muscles secretion are of notthe fullyAfrican understood. hyperoliid frog, Kassina senegalensis, is reported. This peptide represents an entirely novel structuralHere, class the discovery of amphibian of a prototypeskin peptide low and molecular was found mass to display peptide a fromunique the relaxation skin secretion of rat ofartery the Africansmooth hyperoliidmuscle. The frog, peptideKassina was senegalensis named, WL-8, is reported. amide (978 This Da) peptide in accordance represents with an its entirely chain length novel structuraland first and class last of amino amphibian acid skinresidues. peptide We andfurther was de foundscribe to the display structure a unique of its relaxationbiosynthetic of ratprecursor artery smoothidentified muscle. through The peptidemolecular was cloning named, of WL-8 its amideencoding (978 Da)cDNA in accordancefrom a skin with secretion-derived its chain length andcDNA first library. and last amino acid residues. We further describe the structure of its biosynthetic precursor identified through molecular cloning of its encoding cDNA from a skin secretion-derived cDNA library. 2. Results 2. Results Each specimen of K. senegalensis yielded on average 30–35 mg dry weight of skin secretion K. senegalensis followingEach manual specimen stimulation. of Five mg ofyielded lyophilised on average skin secretion 30–35 was mg subjec dry weightted to reverse-phase of skin secretion high followingperformance manual liquid stimulation. chromatography Five mg(RP-HPLC) of lyophilised fractionation skin secretion that produced was subjected a complex to reverse-phasechromatogram high(Figure performance 1). liquid chromatography (RP-HPLC) fractionation that produced a complex chromatogram (Figure1). 1484 1081 678 WL-8 amide Absorbance [mA] Absorbance 275 -128 83:30 91:58 100:25 108:53 117:20 125:48 Time [mm:ss] FigureFigure 1.1. Region of reverse-phase high high performance performance liqu liquidid chromatography chromatography (RP-HPLC) (RP-HPLC) chromatogram chromatogram of oflyophilised lyophilised KassinaKassina senegalensis senegalensis skinskin secretion. secretion. The Theretention retention time timeof the of WL-8 the WL-8 peptide peptide is 97 min is 97 (32.3% min (32.3%acetonitrile) acetonitrile) and indicated and indicated by an arrow. by an arrow. Chromatographic fractions were screened using the smooth muscle bioassay. The single peptide in fraction #97, which was eluted around 32% acetonitrile, exhibiting myotropic activity, was Molecules 2017, 22, 1215 3 of 9 Chromatographic fractions were screened using the smooth muscle bioassay. The single peptide Molecules 2017, 22, 1215 3 of 8 in fraction #97, which was eluted around 32% acetonitrile, exhibiting myotropic activity, was sequenced sequencedunambiguously unambiguously by LC/MS/MS by LC/MS/MS fragmentation fragmentation (Figure2 (Figure). The amino2). The acidamino sequence acid sequence of WL-8 of wasWL- 8determined was determined as: Trp-Met-Ser-Leu-Gly-Trp-Ser-Leu-amide as: Trp-Met-Ser-Leu-Gly-Trp-Ser-Leu-amide (WMSLGWSL). (WMSLGWSL). The WL-8 amide-encoding precursor precursor cDNA cDNA was was successfully cloned cloned from from the skin secretion librarylibrary using using the RACE protocol described. The The open-reading open-reading frame frame consisted consisted of of 76 76 amino acid residuesresidues (Figure 33).). TheThe maturemature peptidepeptide waswas flankedflanked N-terminally by identical propeptide convertaseconvertase processing sitessites probablyprobably involving involving a proteasea protease with with Lys-Arg Lys-Arg cleavage cleavage specificity. specificity. The last The glycyl last residueglycyl residuein the open-reading in the open-reading frame frame is appropriately is appropriately positioned positioned to act to as act an as amide an amide donor donor for for generation generation of ofthe the C-terminal C-terminal leucinamide leucinamide residue residue present present on on the the mature mature peptide. peptide. TheThe nucleotidenucleotide sequence of the WL-8 precursorprecursor has has been been deposited deposited in the in